Typically, when constructed, an offshore platform has the jacket legs sealed to prevent water leakage therein to facilitate towing operations and platform erection. With the jacket legs sealed against water leakage, the offshore platform may either be directly towed to the desired erection site or placed on barges and towed to the erection site. After being towed or transported by means of barges to the erection site, the offshore platform is positioned on the sea bottom by the controlled flooding of one or more of the jacket legs.
Sometimes, it is desirable to pre-install the piles, which are used to anchor the offshore platform to the floor of the body of water in which the platform is to be installed, in the jacket legs of the platform before the platform is transported to the erection site. When the piles are pre-installed in the jacket legs, they must also be sealed against water leakage thereinto.
One type of plug used to seal jacket legs, piles and conductor pipes of an offshore platform is described in U.S. Pat. No. 4,142,371. The plug comprises a steel cup retained within a cylindrical housing the same diameter as the jacket leg or member and is welded therein. The steel cup is retained within the cylindrical housing by means of a molded elastomeric member which has a portion of the cup retrieving cable spirally wrapped within the elastomeric member. To retrieve the cup from the jacket leg or member, a force is applied to the free end of the cable at the top of the jacket leg or member which, in turn, progressively rips the molded elastomeric member apart thereby freeing the steel cup to move upwardly in the jacket leg or member. However, since the plug and the housing must be installed in the jacket leg or member during the construction phase of the platform, any changes subsequent to the installation of the plug and housing which would affect the platform's weight distribution are undesirable to make because they would require relocation of the plug and housing in the jacket leg or member. Also, once the steel cup is removed from the jacket leg or member, since the cable embedded in the elastomeric material merely rips the elastomeric material apart, a residue of elastomeric material remains in the jacket leg or member which usually must be removed therefrom.
Another type of plug used to seal the jacket legs or other members of an offshore platform is an inflatable type plug having a plurality of slips or grippers engaging the jacket leg or member wall anchoring the plug in position by means of an inflatable member therebehind which also seals the interior of the jacket leg or member. An inflatable plug of this type has the disadvantages of the inflatable member rupturing or loosing inflation pressure, the inflatable pressure may not be great enough to cause the slips or grippers to firmly engage the jacket leg or member to hold the plug in position, added equipment must be installed on the platform during towing operations to assure inflation pressure of the inflatable member is maintained and the jacket leg or member may be damaged by the slips or grippers engaged by the same.
Yet another type plug used to seal the jacket leg of an offshore platform is disclosed in U.S. Pat. No. 4,160,612. This type of plug is an inflatable type plug having adjustable locking dogs which engage an annular groove in a locking ring installed on the inside of the jacket leg. An inflatable plug of this type has the disadvantages of requiring a complex adjustment linkage for the locking dogs, the inflatable member may rupture or loose pressure, added equipment must be present to insure the inflation pressure of the inflatable packer is maintained, and a locking ring must be installed inside the jacket leg which may be particularly difficult if the locking ring is not concentric with the jacket leg.
Still yet another type of conductor pipe plug is illustrated in U.S. Pat. No. 4,184,515. This type of plug comprises central body member having elastomeric sealing means and pressure equalization means thereon installed in a cylindrical housing secured to a conductor pipe. The central body member of the plug is retained within the cylindrical housing by means of a plurality of shear pins. While this type of plug is simple to construct and use, for platforms installed in large depths of water the strength of the shear pins required to retain the central body member in the cylindrical housing becomes large requiring very large diameters of cables to be required connecting with the plug for removal from the conductor pipe.
Another type of conductor pipe plug is disclosed in U.S. Pat. No. 4,178,967. This type of plug comprises a central body member having elastomeric sealing means, pressure equalization means, and retractable release means thereon installed in a cylindrical housing secured to conductor pipe. The central body member of the plug is retained within the cylindrical housing by the retractable release means engaging an annular groove in the cylindrical housing. While this plug is simple to manufacture and use, since the retractable release means comprises a plurality of rod like members, each having the outer end thereof engaging the annular groove in the cylindrical housing, for platforms installed in very large depths of water the forces acting on the plug may be so great as to cause the ends of the retractable release means engaging the annular groove in the cylindrical housing to deform the annular groove locally, thereby making it difficult to release the plug during removal operations.
Another type is described in U.S. Pat. No. 2,776,015 and is used in plugging the tubing string in an oil or gas well. This plug comprises a plug body retained in tubing plug sub connected in a tubing string. The plug is retained in the sub by means of a plurality of spheres which engage an annular recess in the sub. Under high pressures, the spheres will be deformed into the annular recess in the sub making it difficult or impossible to release the plug from the sub. Also, the plug lacks a means of indicating when it is allowing fluid flow therethrough or is being released from the sub making it difficult to use.
Another yet different type plug used in offshore platforms is described in U.S. Pat. No. 3,577,737. This plug comprises a compressible rubber member retained between two discs having a central axially disposed rod running therethrough and an upper block assembly slidably receiving the rod therein having a plurality of retractable pins retaining the plug within the cylindrical member in which it is installed on the offshore platform. This type of plug does not have a pressure equalization means and requires a large amount of elastomeric material for the compressible rubber member.